1. Field of the Invention
The present invention relates to current sensors that measure the magnitude of a current, and more particularly, to a current sensor including magnetoresistive elements.
2. Description of the Related Art
Heretofore, in the fields of, for example, electric cars and solar batteries, a current sensor including magnetic detection devices which output an output signal due to an induction field caused by a measurement current has been used. An example of a magnetic detection device included in such a current sensor is a magnetoresistive element such as a giant magnetoresistive (GMR) element.
A GMR element has a basic film structure including an antiferromagnetic layer, a ferromagnetic pinned layer, a non-magnetic material layer, and a free magnetic layer. The ferromagnetic pinned layer is formed on the antiferromagnetic layer so as to be in contact with the antiferromagnetic layer, and has a magnetization direction that is pinned in a certain direction due to the exchange coupling magnetic field (Hex) which occurs between the antiferromagnetic layer and the ferromagnetic pinned layer. The free magnetic layer is stacked on the ferromagnetic pinned layer with the non-magnetic material layer (non-magnetic intermediate layer) interposed between the free magnetic layer and the ferromagnetic pinned layer, and has a magnetization direction which is changed due to the external magnetic field. In a current sensor including GMR elements, the current value of a measurement current is detected using electrical resistance values of the GMR elements which are changed depending on the relationship between the magnetization direction of the free magnetic layer which is changed due to application of the induction field caused by a measurement current and the magnetization direction of the ferromagnetic pinned layer. As a current sensor including GMR elements, a current sensor has been proposed in which a hard bias layer for applying a bias magnetic field to a free magnetic layer is formed to enhance the linear relationship (linearity) between the electrical resistance value of a GMR element and the intensity of the external magnetic field (for example, see Japanese Unexamined Patent Application Publication No. 2006-66821).
In such a current sensor, GMR elements are disposed so as to be oriented in the longitudinal direction of the stripe shape thereof and to be parallel to each other. Hard bias layers which also serve as an electrode are formed at the ends and the midpoint of each of the GMR elements in the longitudinal direction of the stripe shape. Each of the GMR elements is stacked on the hard bias layers, and is electrically connected with the adjacent GMR elements through the hard bias layers disposed at the ends of the GMR element.
To further improve the measurement accuracy in a current sensor, reduction in an output signal offset, reduction in gain variation, and improvement of the linearity of the output characteristics are required. In contrast, a current sensor including magnetoresistive elements has a problem in that an offset occurs in an output signal due to the hysteresis in the R-H curve for a magnetoresistive element. In the current sensor described in Japanese Unexamined Patent Application Publication No. 2006-66821, by applying a bias magnetic field from a hard bias layer to a free magnetic layer, the magnetization direction of the free magnetic layer is initialized so that the hysteresis is reduced.
However, in the current sensor described in Japanese Unexamined Patent Application Publication No. 2006-66821, the free magnetic layer of a GMR element is directly stacked on the hard bias layers. Therefore, in a contact portion which is in the free magnetic layer and which is in contact with a hard bias layer, the magnetization direction is strongly pinned due to the bias magnetic field from the hard bias layer. Even when the induction field from a measurement current operates, the magnetization direction is not changed, causing the contact portion to serve as a dead zone from which an output signal is not obtained. Thus, when a dead zone is present, there arises a problem in that the hysteresis is not sufficiently reduced.